1. Field of the Invention
The present invention relates to a liquid crystal display device and, more particularly, to a liquid crystal display device which has a wide field angle and can be easily manufactured and driven.
2. Description of the Related Art
As a liquid crystal display device, a liquid crystal display device using a TN (Twisted Nematic) mode or an STN (Super Twisted Nematic) mode is generally used.
FIG. 1 is a sectional view showing a conventional liquid crystal display device using a TN or STN mode, and FIG. 2 shows the aligned states of liquid crystal molecules in the presence of an electric field.
The liquid crystal display device in FIG. 1 is constituted by a pair of transparent substrates 1 and 2; a liquid crystal 9 held between the substrates 1 and 2; and a pair of polarizing plates 14 and 15.
Pixel electrodes 3 are formed on a surface, of the substrate 1, opposing the liquid crystal 9, and an aligning film 5 is formed on the substrate including the pixel electrodes 3.
Black masks 7 for shielding leakage light are formed on a surface, of the substrate 2, opposing the liquid crystal 9, and a transparent insulating film 8 is formed on the substrate including the black masks 7. A counter electrode 4 is formed on the transparent insulating film 8, and an aligning film 6 is formed on the counter electrode 4.
The surfaces of the aligning films 5 and 6 are subjected to an aligning process in directions offset from each other at a predetermined angle (almost 90.degree. in the TN mode, 180.degree. to 270.degree. in the STN mode).
The liquid crystal 9 consists of a nematic liquid crystal added with a chiral agent, and, in the absence of an electric field, molecules 9a of the liquid crystal 9 are aligned in a twisted state having a predetermined pre-tilt angle. When an electric field is applied across the pixel electrodes 3 and the counter electrode 4, as shown in FIG. 2, the liquid crystal molecules 9a are aligned to be vertical to the opposite surfaces of the transparent substrates 1 and 2 in accordance with the strength of the applied electric field.
When the aligned state of the liquid crystal molecules 9a changes, the refractive anisotropy .DELTA.n of the liquid crystal 9 with respect to an incident light changes, and a value .DELTA.n.d (product between the refractive anisotropy .DELTA.n and a layer thickness d of the liquid crystal 9) changes accordingly. For this reason, the polarized state of a linearly polarized light passing through the polarizing plate 15 on the incident side changes in accordance with the value .DELTA.n.d of the liquid crystal 9, and thus the intensity of light emerging from the polarizing plate 14 on an exit side changes.
For this reason, when the aligned state of the liquid crystal molecules 9a is controlled by adjusting the applied voltage, an image can be displayed by controlling the transmission and shielding of respective pixel regions with respect to light.
In the liquid crystal display device in which liquid crystal molecules are twisted and aligned as in the TN or STN mode, the gradation level of a display image largely changes depending on an observation direction. For this reason, an angle range in which the display image can be normally observed is small. That is, a field angle is disadvantageously narrow.
A reason why the field angle is narrow will be described below in detail with reference to FIG. 2.
Referring to FIG. 2, reference symbols IA, IB, and IC denote light beams incident on the liquid crystal display device, and reference symbols OA, OB, and OC denote exit light beams corresponding to the incident beams IA, IB, and IC, respectively. When the display image of the liquid crystal display device is observed from a direction almost perpendicular to the screen of the liquid crystal display device, the pixels are displayed by the exit beam OA. When the display image of the liquid crystal display device is observed from the lower right direction of the liquid crystal display device, pixels are displayed by the exit beam OB. When the display image of the liquid crystal display device is observed from the lower left direction of the liquid crystal display device, the pixels are displayed by the exit beam OC.
The tilt angle of the parallel axis of the liquid crystal molecules 9a changes with respect to the incident beams IA, IB, and IC. Thus, different values .DELTA.n are obtained with respect to the incident beams IA, IB, and IC. Different apparent liquid crystal layer thicknesses d are obtained with respect to the incident beams IA, IB, and IC. For this reason, the value .DELTA.n.d with respect to the incident beam IA, the value .DELTA.n.d with respect to the incident beam IB, and the value .DELTA.n.d with respect to the incident beam IC, i.e., values .DELTA.n.d(IA), .DELTA.n.d(IB), and .DELTA.n.d(IC), are different from each other.
The intensity of a light beam emerging from the polarizing plate 14 on the exit side is dependent on the value .DELTA.n.d. For this reason, the brightness of each pixel changes depending on the incident angle of a beam.
As described above, in the conventional liquid crystal display device, the brightness of each pixel is dependent on an observation direction. Therefore, the observation direction in which a display image can be observed as an image having good contrast between a bright portion and a dark portion is limited, and a narrow field angle is obtained.
As liquid crystal display devices, a positive display type liquid crystal display device in which the polarizing plates 14 and 15 are formed to cause the liquid crystal molecules 9a to shield a beam when the liquid crystal molecules 9a are aligned in a direction perpendicular to the substrate surfaces; and a negative display type liquid crystal display device in which the polarizing plates 14 and 15 are formed to cause the liquid crystal molecules 9a to transmit light when the liquid crystal molecules 9a are aligned in a direction perpendicular to the substrate surfaces are known. In the positive display type liquid crystal display device, a dark portion becomes bright due to a change in incident angle of a beam, thereby degrading the contrast between the dark portion and a bright portion. In the negative display type liquid crystal display device, a bright portion becomes dark due to a change in incident angle of a beam, thereby degrading the contrast between the bright portion and a dark portion.
As a scheme for solving a problem of a narrow field angle, an alignment control scheme and a voltage control scheme are conventionally proposed.
According to the alignment control scheme, one or both of the substrates of a liquid crystal display device are subjected to an aligning process such that liquid crystal molecules are partially aligned at different pre-tilt angles. In this manner, the initial aligned states (twisted and aligned states) of the liquid crystal molecules are made different from each other in regions of each pixel.
In the voltage control scheme, the electrodes of one substrate of a liquid crystal display device are constituted by a plurality of electrodes in units of pixels. Different drive voltages are applied across a plurality of electrodes of each pixel and the electrode of the other substrate, respectively. In this manner, the strength of an electric field applied to the liquid crystal is partially changed to change the tilt angles of the liquid crystal molecules in units of the regions of each pixel.
More specifically, in each of the alignment control scheme and the voltage control scheme, the aligned states of liquid crystal molecules are partially changed in each pixel. In this manner, changes in value .DELTA.n.d with respect to a change in incident angle of a beam are different from each other in units of the regions of each pixel. For this reason, even when the incident angle of a beam changes, the average of the values .DELTA.n.d slightly changes. Therefore, dependence of pixel brightness on an observation direction is reduced, and a field angle is widened.
However, according to the alignment control scheme, an aligning process for partially aligning liquid crystal molecules at different pre-tilt angles is not easily performed. Therefore, the alignment control scheme cannot be easily utilized in practice.
According to the voltage control scheme, a conventional process may be performed as an aligning process, and a plurality of electrodes can be formed in each pixel by the present photolithographic technique. For this reason, the voltage control scheme can be utilized in practice. However, according to the voltage control scheme, since drive signals having different voltages must be supplied to divided electrodes, respectively, drive control of a liquid crystal display device becomes complex.